Solid-state relays perform functions similar to electromagnetic relays, but are more reliable, since there are no moving parts. Since the turn on and turn off times of a solid-state relay are controllable, the solid-state relay also minimizes the generation of switching transients.
A preferred semiconductor device for power control in a solid-state relay is the insulated gate FET (Field Effect Transistor) because of its high power gain. FETs used for power switching use are usually enhancement mode types. This means that they are normally non-conducting. When a gate voltage above a threshold is applied, the FET becomes conducting. FETs are available in two gate polarities; N channel and P channel.
In an FET, current flows along a semiconductor path called the channel. At one end of the channel, there is a source electrode, and at the other end, a drain electrode. The physical diameter of the channel is fixed, but its effective electrical diameter is changed by applying voltage to a gate electrode. The conductivity of the FET depends, at any given time, on the electrical diameter of the channel. A small change in gate voltage can cause a large variation in current from the source to the drain. In this way, the FET switches current on or off.
Typically, FETs used for power switching are enhancement mode types, that is, they are normally non-conducting. When a gate voltage above a certain threshold is applied, the FET becomes conducting. Such FETs are used to control current flow and are available in two gate polarities; N channel and P channel.
Solid state relays perform functions similar to electromagnetic relays, but are more reliable, since there are no moving parts. Since the turn on and turn off times of a solid state relay are controllable, the solid state relay also minimizes the generation of switching transients.
Solid state relays are used in spacecraft, satellites and in high energy physics instrumentation. In these specific applications, the solid state relays are subjected to many forms of radiation damage by the surrounding environment.
A preferred semiconductor device for power control in a solid state relay is the insulated gate FET (Field Effect Transistor) because of its high power gain. FETs used for power switching use are usually enhancement mode types. This means that they are normally non-conducting. When a gate voltage above a threshold is applied, the FET becomes conducting. FETs are available in two gate polarities; N channel and P channel.
Power switching circuits designed for general purpose use are usually constructed with N channel FETs because, for any given die size transistor, the N channel FET has a lower on resistance than a correspondingly sized P channel FET would have.
Present art for radiation hardened solid state relay circuits use specially designed radiation hardened N channel FETs for power switching functions. The principal benefit of these radiation hardened N channel FETs parts is that the gate threshold voltage doesn't change much after being exposed to radiation. However, these parts have limited sources of supply, are expensive and may have long lead times, leading to higher prices and longer delivery times for the radiation tolerant circuits that incorporate these types of parts.
Normally, the performance of non-radiation hardened FETs when exposed to radiation, if conventional non-radiation hardened N Channels FETs are used, in switching applications where radiation is present, the function tends to fail at relatively low radiation levels because the gate threshold voltage of the N channel FET shifts more negatively with accumulated radiation dose, and ultimately falls close to zero. At this point, the N channel FET conducts current with little or no gate voltage applied. Therefore, the part is difficult to control.
Additionally, the gate threshold voltage of a conventional, non-radiation hardened P channel FET also shifts more negative as it is exposed to accumulated radiation dose. However, the initial threshold voltage is negative. Therefore, the gate threshold voltage never goes through a region where the FET is uncontrollable, it only goes from a negative value to a more negative value. Therefore, conventional P channel FETs can be more immune to total dose effects than conventional N channel FETs if the proper gate drive signal is provided.